This invention relates to apparatus for detecting bent, folded or broken portions of printing sheets immediately after commencing the printing operation.
Referring to FIG. 1 which shows a prior art sheet printing press, a printing sheet 1 fed from a sheet feeder 2 is stopped by a front register 3, and the position of the sheet is determined by the front register and side registers, not shown, by being drawn by a lateral needle 4. Then the sheet is clamped by jaws of a swinger 5 to be transferred to an impression cylinder 6, and then printed with a first color by a blanket cylinder 8 which is supplied with the printing ink from a plate cylinder 7. Then the printed sheet is transferred to a second printing unit comprising identical rollers through an intermediate cylinder 9 to be printed with a second color. After being printed with a plurality of colors, the printed sheet is transferred to a receiving cylinder 11 via a transfer cylinder 10 and then conveyed to a delivery device 13 by being clamped by jaws of a conveyor chain 12.
As shown in FIG. 2, a printing sheet 2 collides against front registers 3 to correctly position its front edge and is laterally drawn by a lateral needle 4, not shown in FIG. 2, to correct its lateral position. Usually, an incorrectly fed sheet is detected by a photoelectric detector, located a little short of the front registers 3. The photoelectric detector comprises two light emitting elements 14 located beneath the sheet 2 near its opposite sides and two light receiving elements 15 disposed oppositely to the light emitting elements 14. With this arrangement, since the light emitted by the light emitting elements 14 reaches the light receiving elements 15, when two superposed sheets are delivered simultaneously such improper sheet feeding alters the quantity of light transmitted. When the front edge of the sheet 2 does not reach the front registers 3, the light would be directly received by the light receiving elements 15 thus enabling them to detect improperly supplied sheet. Upon detection of such improperly fed sheet, the pay out operation of the sheet feeder 1 is stopped while at the same time the printing operation of the printing cylinder is prevented by displacing the same to an inoperative position, thus preventing improper printing.
Such photoelectric detectors can also detect a state in which one or both front corners of the sheet are drastically bent or folded back. However, since the photoelectric detectors are generally located at positions substantially remote from both side edges of the sheet, they can not detect a state in which front corners are bent or folded back only slightly. Even when three or more detectors are provided for a printing press, since the detectors are located apart from the side edges of the sheet, it is also impossible to detect slightly bent or folded back states at the corner.
The bent or folding back states at the corners are liable to be formed when sheets are stacked into another stack or when an automatic device for counting the number of sheets is used. Especially, as the sheet number counter sequentially turns up the corners of stacked sheets with a mechanical device, the chance of bending up the corners increases. Such corner bending up also occurs at the corners of the rear edge of the sheet. Further, the sheet often breaks at or is formed with notches intermediate points along the side edges thereof.
FIGS. 3a, 3b and 3c show abnormal states along the side edge of a sheet. In these Figures, arrows shown the direction of movement of the sheet. FIG. 3a shows a small bent up or folded back portion at the front end of one side, FIG. 3b shows a small bent up or folded back portion at the rear end of the same side, and FIG. 3c shows a small notch at an intermediate point of the same side.
Since these abnormal states can not be detected by the prior art photoelectric detectors of the type described above, such defective sheets would be printed thus forming defective printed sheets. Consequently it is necessary to visually examine completed books so as to check whether or not they contain defective printed sheets. This is not only troublesome but also books containing defective printed pages must be discarded. In an offset printing press the folded back portion applies a local excessive pressure to the blanket so that such local excessive pressure results in nonuniform printings for all succeeding sheets unless the blanket is replaced by a new one.